Technical Field
The present invention relates to an optical unit that generates interference light and a displacement measuring device that measures displacement by detecting that interference light.
Background Art
Patent Document 1, for example, discloses a critical dimension measuring device that uses optical interference. According to this critical dimension measuring device, parallel beams that are emitted from a light source and traverse a half-mirror are incident on a diffraction grating, which splits the beams into zero-order light that traverses the diffraction grating and ±pth-order diffracted light. The zero-order light and ±pth-order diffracted light are each reflected by mirrors disposed at prescribed positions. Each beam of reflected light is then reflected by a half-mirror and received by a light-receiving element (see paragraphs [0036] and [0037] of the specification and FIG. 1 in Patent Document 1, for example).
Patent Document 2, meanwhile, discloses a displacement measuring device that measures the relative displacement of a pair of diffraction gratings. According to this displacement measuring device, the relative displacement of the pair of diffraction gratings corresponds to the intensity of interference light between beams of diffracted light emitted from one of the diffraction gratings in the stated pair of diffraction gratings. The displacement can be measured by detecting that intensity.